1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor element used in the field of electronics and optoelectronics for example.
2. Background Art
AlGaInN has a wide bandgap and is therefore used as a material for semiconductor elements of a high withstand voltage and semiconductor elements capable of emitting light of short wavelengths. AlGaInN is epitaxially grown on a substrate formed of sapphire, SiC or Si by metal organic chemical vapor deposition (MOCVD). As the substrate, SiC that highly lattice—matches to GaN and has high heat conductivity is ordinarily used.
Japanese Patent Laid-Open No. 2013-012767 discloses a chemical compound semiconductor device having an AlN layer epitaxially grown on a substrate and a nitride semiconductor layer epitaxially grown on the AlN layer, irregularities in the interface between the AlN layer and the nitride semiconductor layer being larger than those in the interface between the substrate and the AlN layer.
Increasing irregularities in the interface causes an apprehension of an increase in leak current. Therefore, the art disclosed in Japanese Patent Laid-Open No. 2013-012767 includes forming an AlGaN layer or a GaN layer having Fe added thereto (doped with Fe) between the AlN layer and the nitride semiconductor layer.
In the case of limiting the leak current by AlGaN, however, there is a need to select x satisfying 0<x≦0.1 for AlxGa1-xN in order to avoid reducing the heat conductivity. With a layer of such a low Al composition ratio, a sufficiently high resistance value cannot be obtained. That is, the leak current cannot be controlled with AlGaN.
In the case where a GaN layer to which Fe is added is formed between the AlN layer and the nitride semiconductor layer, the GaN layer has a high resistance since n-type carriers can be compensated with Fe, thereby enabling leak current control. However, the GaN layer to which Fe is added has increased crystal grain boundaries and has its surface roughened, since irregularities exist in the surface of AlN layer, and since the GaN layer to which Fe is added can easily grow three-dimensionally. The crystal grain boundaries become dislocations and cause degradation of the crystallinity of a portion including the channel layer, thereby reducing the output and withstand voltage of the semiconductor element. Roughening of the surface means that surface opacity is easily caused by a change in an epi-condition with a slight environmental change such as in a chamber. That is, a margin of the epi-condition is considerably small.